fix: save does not return id, save does not return generated

[TheBlue-1]

Description of change

Generated fields got overridden by undefined in the entity returned by save, if there are one to many relations in the entity.
I changed this, so the generated fields get merged and not overriden.
Fixes #5520
Fixes #2422
Fixes #4090

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• [not needed] Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

Thank you for contribution! Can you please include tests to this PR?

[Berbass]

Hey,
This fix will save my life!

Will it be merged soon?

[pleerock]

Thank you for contribution!